1. Field of the Invention
The present invention relates to a thermal type flow meter for measuring a mass flow rate of a fluid, and in particular relates to a thermal type flow meter suitable for detecting a flow rate of an exhaust gas from an engine.
2. Description of the Related Art
As a thermal type measurement system, firstly, there is known a thermal type flow measuring device in which: two resistors are arranged in a fluid, one of them is used as a temperature measuring resistor for detecting the temperature of the fluid while the other one is used as a heating resistor for detecting the flow rate of the fluid; and the mass flow rate of the fluid is measured by keeping a temperature difference (ΔTh) between these resistors always constant (for example, see International Patent Publication No. WO 03/016833 (hereinafter called Patent Document 1)).
As the temperature measuring resistor, for example, a platinum wire or the like having a large variation in resistance value depending on the temperature is used. Since there is a certain relationship between the temperature and resistance value of the platinum wire, the temperature of the temperature measuring resistor can be detected by measuring the resistance of the temperature measuring resistor. Furthermore, the temperature of the fluid can be obtained assuming that the temperature of peripheral fluid and the temperature of the temperature measuring resistor are equal.
However, when the fluid temperature abruptly changes, the temperature of the temperature measuring resistor and the fluid temperature become different due to the heat capacity and the like of the temperature measuring resistor. Consequently, the fluid temperature cannot be detected correctly and an error in a flow rate increases.
In contrast, secondly, there is known a flow rate measuring device in which: a temperature measuring resistor is not used but two heating resistors are disposed in a fluid; and a flow rate unrelated to the temperature of the fluid is detected from a relationship between amounts of heat supplied from the respective heating resistors to the fluid (for example, see Japanese Patent Application Publication No. Sho 59-136620 (hereinafter called Patent Document 2)).
According to this method, a temperature measuring resistor is not used and therefore accurate flow rate measurement is possible even if the fluid temperature abruptly changes.